Magneto-optical recording medium

ABSTRACT

A magneto-optical recording medium capable of direct overwriting in a light intensity modulation mode comprises a magnetic multilayer structure comprising, in order form an surface side thereof, a readout layer R 01 , a memory layer M 1 , an exchange force control layer C 12  and a recording layer W 2 , four layers in all. The magnetic multilayer structure satisfies TC W2  &gt;Tc M1 , t R01  /(t R01  +t M1 )=0.3 to 0.6, and t R01  +t M1  =20 to 40 nm where Tc M1  is a Curie temperature of M 1 , Tc W2  is a Curie temperature of W 2 , t R01  is a thickness of R 01 , and t M1  is a thickness of M 1 . R 01  contains Gd, Fe, and Co as main components with the proviso that Gd is contained in an amount of 23 to 27 at %, M 1  contains Tb, Fe, and Co as main components with the proviso that Tb is contained in an amount of 21 to 25 at %, and C 12  contains Gd, Fe, and Co as main components with the proviso that Gd is contained in an amount of 23 to 32 at %, and has a thickness of 30 nm or below.

BACKGROUND OF THE INVENTION

The present invention relates to a magneto-optical recording medium capable of direct overwriting in a light intensity modulation mode.

A magneto-optical (MO) recording medium is a recording medium wherein a magnetic thin film is locally heated by laser light or the like, and the magnetization direction of heated domains is reversed by an external magnetic field for recording, and these recorded domains having varying magnetization directions are read out by the Kerr effect, and the Faraday effect.

One feature of the magneto-optical recording medium is that recording density can be increased, and another feature is that, unlike a hard disk that is a mass storage magnetic recording medium, a medium replacement is easily achievable. However, a grave problem with an ordinary magneto-optical recording medium is that rewriting is slow, generally because overwriting cannot be used for rewriting; new information must be recorded after erasure of recorded information.

Magneto-optical recording media capable of direct overwriting by light intensity modulation (which may hereinafter be often called light intensity modulation overwriting), for instance, are disclosed in JP-A 62-175948, JP-B's 8-16993 and 8-16996, etc. For these magneto-optical media, however, it is required to built an initializing magnet in a driving device. On the other hand, magneto-optical recording media which is capable of light intensity modulation overwriting without recourse to any initializing magnet are disclosed in WO 90/02400, Japanese Patent No. 2503708, JP-A 6-12711, etc.

SUMMARY OF THE INVENTION

An object of the invention is to provide a magneto-optical recording medium capable of direct overwriting in a light intensity modulation mode, wherein C/N is improved with a stable storage of recorded information.

The aforesaid object is achievable by the inventions defined below as (1) to (5).

(1) A magneto-optical recording medium capable of direct overwriting in a light intensity modulation mode, which comprises a substrate and a magnetic multilayer structure located on a surface side thereof, wherein:

said magnetic multilayer structure comprises, in order from a substrate side thereof, a readout layer R₀₁, a memory layer M₁, an exchange force control layer C₁₂ and a recording layer W₂,

each of said magnetic layers containing a rare earth element and a transition element,

said readout layer R₀₁, said memory layer M₁, and said recording layer W₂ having perpendicular magnetic anisotropy at room temperature, and

adjacent magnetic layers being mutually coupled with exchange force,

said magnetic multilayer structure satisfying the following conditions:

    Tc.sub.W2 >Tc.sub.M1

    t.sub.R01 /(t.sub.R01 +t.sub.M1)=0.3 to 0.6

    t.sub.R01 +t.sub.M1 =20 to 40 nm

where Tc_(M1) is a Curie temperature of said memory layer M₁, Tc_(W2) is a Curie temperature of said recording layer W₂, t_(R01) is a thickness of said readout layer R₀₁, and t_(M1) is a thickness of said memory layer M₁,

said readout layer R₀₁ containing gadolinium, iron, and cobalt as main components with the proviso that gadolinium is contained in an amount of 23 to 27 at %,

said memory layer M₁ containing terbium, iron, and cobalt as main components with the proviso that terbium is contained in an amount of 21 to 25 at %, and

said exchange force control layer C₁₂ containing gadolinium, iron, and cobalt as main components with the proviso that gadolinium is contained in an amount of 23 to 32 at %, and having a thickness of 30 nm or below.

(2) The magneto-optical recording medium of (1), wherein said recording layer W₂ contains dysprosium, iron, and cobalt as main components.

(3) The magneto-optical recording medium of (1), wherein said magnetic multilayer structure has a thickness of 80 nm or below.

(4) The magneto-optical recording medium of (1) or (2), wherein:

said magnetic multilayer structure further comprises, in order from a surface side of said recording layer W₂, a switching layer S₃ and an initializing layer I₄, each being a magnetic layer containing a rare earth element and a transition element and having perpendicular magnetic anisotropy at room temperature,

adjacent layers in said magnetic multilayer structure are mutually coupled with exchange force, and

said magnetic multilayer structure satisfy simultaneously the following conditions:

    Tc.sub.I4 >Tc.sub.W2 >Tc.sub.M1

    TC.sub.I4 >Tc.sub.W2 >Tc.sub.S3

where Tc_(M1) is the Curie temperature of said memory layer M₁, Tc_(W2) is the Curie temperature of said recording layer W₂, Tc_(S3) is a Curie temperature of said switching layer S₃, and Tc_(I4) is a Curie temperature of said initializing layer I₄.

(5) The magneto-optical recording medium of (4), wherein said switching layer S₃ contains terbium, and iron as main components, and said initializing layer I₄ contains terbium, and cobalt as main components.

By limiting the content of gadolinium or Gd in the readout layer R₀₁ to the aforesaid range according to the invention, it is possible to enhance the Kerr effect and increase carrier signals.

By limiting the content of terbium or Tb in the memory layer M₁ to the aforesaid range, it is possible to enhance the coercive force of the memory layer M₁ and, hence, store recorded information in a stabilized state.

By limiting the thickness ratio t_(R01) /(t_(R01) +t_(M1)) to the aforesaid range, it is possible to keep coercive force high on condition that the readout layer R₀₁ and memory layer M₁ are integrated with each other by exchange coupling and, hence, stabilize storage of recorded information. By limiting this thickness ratio to the aforesaid range, it is also possible to achieve sufficient enhancement of the Kerr effect and, hence, obtain high-enough C/N.

The exchange force control layer C₁₂ is provided to control exchange forces between the readout layer R₀₁ and the recording layer W₂, and between the memory layer M₁ and the recording layer W₂. By using gadolinium or Gd, iron or Fe and cobalt or Co as the main components of the exchange force control layer C₁₂ and using Gd in the aforesaid amount on the premise that the Tb content of the memory layer M₁ is in the aforesaid range and the thickness t_(R01) +t_(M1) is in the aforesaid range, it is possible to increase carrier signals with a noise decrease and, consequently, obtain high C/N.

In this regard, JP-A 7-29231 discloses a magneto-optical recording medium which comprises at least four layers, i.e., a read layer, a memory layer, an exchange coupling force regulating layer and a writing layer, and which is capable of light intensity modulation overwriting. The read layer, memory layer, exchange coupling force regulating layer and writing layer set forth therein are magnetic layers similar to the readout layer R₀₁, memory layer M₁, exchange force control layer C₁₂ and recording layer W₂ in the present invention, respectively. However, nowhere therein are the aforesaid limitation conditions of the invention described whatsoever.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the present invention will be apparent from the following description and drawings, in which:

FIG. 1 is a schematic representation of one exemplary construction of the magneto-optical recording medium according to the invention.

FIG. 2 is a schematic representation of another exemplary construction of the magneto-optical recording medium according to the invention.

FIG. 3 is one illustration of overwriting on the magneto-optical recording medium according to the invention.

FIG. 4 is another illustration of overwriting one the magneto-optical recording medium according to the invention.

FIG. 5 is yet another illustration of overwriting on the magneto-optical recording medium according to the invention.

FIG. 6 is a further illustration of overwriting on the magneto-optical recording medium according to the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Two embodiments I and II of the magneto-optical recording medium according to the present invention are explained.

Embodiment I (FIGS. 1, 3, and 4)

Embodiment I of the magneto-optical recording medium is shown in FIG. 1. In this magneto-optical recording medium embodiment, a magnetic multilayer structure is provided on the surface side of a substrate. The magnetic multilayer structure comprises, in order from a substrate side thereof, a readout layer R₀₁, a memory layer M₁, an exchange force control layer C₁₂ and a recording layer W₂, four layers in all. On the back side of the magnetic multilayer structure, i.e., between the substrate and the magnetic multilayer structure there is provided a first dielectric layer. On the uppermost side of the magnetic multilayer structure there is a second dielectric layer having a heat radiating layer.

Each of the magnetic layer forming the magnetic multilayer structure of FIG. 1 is made up of an amorphous alloy containing a rare earth element and a transition element, and has perpendicular magnetic anisotropy at room temperature. Adjacent magnetic layers in the multilayer structure are mutually coupled with exchange force.

To perform the light intensity modulation overwriting to be described below, it is required to satisfy the following condition:

    Tc.sub.W2 >Tc.sub.M1

where Tc_(M1) is the Curie temperature of the memory layer M₁ and TC_(W2) is the Curie temperature of the recording layer W₂.

Light Intensity Modulation Overwriting

How to perform the light intensity modulation overwriting on the magneto-optical recording medium having such construction as shown in FIG. 1 is explained with reference to FIGS. 3, and 4. For simplicity of illustration, only the memory layer M₁ and recording layer W₂ are shown in FIGS. 3, and 4. The readout layer R₀₁ is a magnetic layer for enhancement of read signals, and so has no intermediate influence on the action of the light intensity modulation overwriting. The exchange force control layer C₁₂, too, has no intermediate influence on the action of the light intensity modulation overwriting, because it behaves as a magnetic domain wall between the memory layer M₁ and the recording layer W₂.

In both FIGS. 3 and 4, the condition Tc_(W2) >Tc_(M1) is satisfied.

In FIGS. 3, and 4, let [0] indicate a recording state where magnetic domains having upward magnetization are recorded in the recording layer M₁, and let [1] indicate a recording state where magnetic domains having downward magnetization are recorded therein.

A medium capable of overwriting is placed in the recording state [0] or [1]. FIG. 3 is illustrative of the case where, on overwriting, the medium is put in the recording state [1] irrespective of the initial recording state, and FIG. 4 is illustrative of the case where, on overwriting, the medium is put in the recording state [0] irrespective of the initial recording state.

In FIGS. 3, and 4, a white arrow with a small black arrow inside indicates the direction of magnetization of the whole magnetic layer while a black arrow indicates the direction of magnetization of a transition element subordinate lattice in the magnetic layer. In this embodiment, both the memory layer M₁ and the recording layer W₂ have a compensation temperature lower than room temperature, so that the direction of magnetization of the whole magnetic layer is always in alignment with the direction of magnetization of the transition element subordinate lattice.

Referring to FIG. 3, an upward initializing magnetic field Hi is applied on the magnetic multilayer structure to keep the magnetization of the recording layer W₂ upward in FIG. 3. The recording layer W₂ has no influence on the magnetization of the memory layer M₁, because its coercive force at room temperature is relatively lower than that of the memory layer M₁, and so only the direction of magnetization of the recording layer W₂ is in alignment with the direction of the initializing magnetic field Hi.

Then, the magnetic multilayer structure is irradiated with a laser beam to bring the temperature of the area of the magnetic multilayer structure irradiated up to Tc_(W2) or higher, thereby allowing the magnetizations of the memory layer M₁ and recording layer W₂ to vanish. As the laser beam moves, the temperature of the magnetic multilayer structure decreases. Upon the temperature falling below Tc_(W2), the recording layer W₂ is magnetized downward by a downwardly applied bias magnetic field Hb, so that the magnetization of the recording layer W₂ takes a downward direction. It is here to be noted that the bias magnetic field Hb is always applied for overwriting. Upon the temperature decreasing further and falling below Tc_(M1), the magnetization of the memory layer M₁ takes a downward direction by the exchange coupling force between the memory layer M₁ and the recording layer W₂, and so the magnetic multilayer structure is placed in the recording state [1].

The foregoing are also true of FIG. 4. First, an upward initializing magnetic field Hi is applied on the magnetic multilayer structure to keep the magnetization of the recording layer W₂ upward in FIG. 4. Then, the magnetic multilayer structure is irradiated with a laser beam whose power is lower than is the case with FIG. 3 to heat the magnetic multilayer structure to a temperature higher than Tc_(M1) and below Tc_(W2), thereby allowing the magnetization of the memory layer M₁ to vanish. Then, as the temperature of the magnetic multilayer structure is below Tc_(M1), the magnetization of the transition element subordinate lattice in the memory layer M₁ takes an upward direction by the exchange force between the memory layer M₁ and the recording layer W₂, and so the magnetic multilayer structure is placed in the recording state [0]. In FIG. 4, too, the bias magnetic field Hb is always applied, as in the case of FIG. 3, for the purpose of averting the on and off of the bias magnetic field Hb. Thus, the bias magnetic field Hb has no influence on the overwriting process of FIG. 4.

After overwriting, the recording layer W₂ is magnetized downward in the case of FIG. 3 for high power recording, and upward in the case of FIG. 4 for low power recording. In either case, the initializing magnetic field Hi is applied before the next overwriting, whereby the transition element subordinate lattice in the recording layer W₂ is magnetized upward; that is, the direction of magnetization of the recording layer W₂ just before overwriting is independent of hysteresis. By irradiating the magnetic multilayer structure with a laser beam of high power or lower just after initialization, therefore, it is possible to perform overwriting repeatedly. In other words, the light intensity modulation overwriting becomes feasible.

As can be see from FIGS. 3 and 4, each magnetic layer has the following role. The memory layer M₁ is a magnetic layer for storing the information to be read using the Kerr effect. The recording layer W₂ is a magnetic layer that plays a role in the magnetization of the memory layer M₁ by exchange coupling force, and determines the direction of magnetization of the memory layer M₁.

For simplicity of illustration, the magnetic multilayer structure is assumed to be heated to Tc_(W2) or higher in the case of FIG. 3. However, if the recording layer W₂ can practically be in alignment with the direction of the bias magnetic field, the objective temperature can then be below Tc_(W2). Likewise, the magnetic multilayer structure is assumed to be heated to a temperature that is higher than Tc_(m1) and below Tc_(W2) in the case of FIG. 4. However, if the magnetization of the recording layer W₂ can be transferred to the memory layer M₁, the objective temperature can then be below Tc_(M1).

Magnetic Multilayer Structure

In the present invention, the magnetic multilayer structure in the magneto-optical recording medium capable of the light intensity modulation overwriting is constructed as follows.

Readout Layer R₀₁

The readout layer R₀₁, to be coupled to the memory layer M₁ by exchange force, is provided to achieve C/N improvements.

The readout layer R₀₁ is made up of an amorphous alloy composed mainly of Gd, Fe, and Co. The Gd content of the readout layer R₀₁ is in the range of 23 to 27 at %, and preferably 24 to 26 at %. Too much or little Gd gives rise to a Curie temperature drop and, hence, a C/N drop. The readout layer R₀₁ has an atomic ratio, Fe/(Fe+Co), of 0.65 to 0.75, and preferably 0.68 to 0.73. Too high or low an atomic ratio results in a decrease in the Kerr effect due to a Curie temperature drop and, hence, a C/N drop.

The thickness of the readout layer R₀₁ may be found from the following conditions:

    t.sub.R01 /(t.sub.R01 +t.sub.M1)=0.3 to 0.6

    t.sub.R01 +t.sub.M1 =20 to 40 nm

Here t_(R01) is the thickness of the readout layer R₀₁, and t_(M1) is the thickness of the memory layer M₁. When t_(R01) /(t_(R01) +t_(M1)) is too small, the Kerr effect becomes slender, resulting in a C/N drop, and when t_(R01) /(t₀₁ +t_(M1)) is too large, the combined coercive force of the readout layer R₀₁ and memory layer M₁ decreases, resulting in unstable storage of recorded information. When t_(R01) +t_(M1) is too small, the Kerr effect becomes slender, resulting in a C/N drop, and when t_(R01) +t_(M1) is too large, a post-overwriting C/N drops due to insufficient transfer of magnetization of the recording layer to the memory layer.

Memory Layer M₁

The memory layer M₁ contains Tb, Fe, and Co as main components. The Tb content of the memory layer M₁ is in the range of 21 to 25 at %, and preferably 21 to 23 at %. Too little or much Tb results in too low a coercive force or Curie temperature. The memory layer M₁ has an atomic ratio, Fe/(Fe+Co), of preferably 0.85 to 0.95, and more preferably 0.88 to 0.92. Too low an atomic ratio results in too high a Curie temperature whereas too high an atomic ratio leads too low a Curie temperature.

The thickness of the memory layer M₁ may be determined on the basis of its relation to the thickness of the readout layer R₀₁, as already mentioned.

Preferably, the memory layer M₁ contains a non-magnetic element. By adding the non-magnetic element to the memory layer, it is possible to achieve C/N improvements because of an output increase. Preferably but not exclusively, at least one non-magnetic element is selected from Cr, Ti, Ta, Mo, W, V, Zr, Nb, Al, etc. In view of corrosion resistance improvements and cost, however, it is preferable to use Cr in combination with other element, with the sole use of Cr being most preferred.

Exchange Force Control Layer C₁₂

The exchange force control layer C₁₂ is provided to control the exchange force between the memory M₁ and the recording layer W₂. To this end, a layer of any desired construction may be used with proviso that it has such an effect. However, the following layers are preferred in the practice of the invention.

(1) A layer containing Gd, Fe, and Co as main components, with the easy axis of magnetization lying in its own plane direction at room temperature, wherein the temperature at which the easy axis of magnetization takes a vertical direction is found in the range of higher than 100° C. to the Curie temperature of the exchange force control layer C₁₂.

(2) A layer containing Gd, Fe, and Co as main components, with the easy axis of magnetization lying in its own plane direction.

Of these layers, particular preference is given to (1). When the exchange force control layer C₁₂ is made up of layer (1), the easy axis of magnetization changes direction in the vicinity of its compensation temperature. Transfer of magnetization from the recording layer W₂ to the memory layer M₁ by exchange force is explained with reference to FIGS. 3, and 4. This transfer of magnetization takes place easily, because the easy axis of magnetization of the exchange force control layer C₁₂ is in the vertical direction. On the other hand, the operation for a reversal of magnetization of the recording layer W₂ by the application of the initializing magnetic field Hi to the magnetic multilayer structure in the recording state [1] is carried out at room temperature. Since, at this time, the easy axis of magnetization of the exchange force control layer C₁₂ is in its own plane direction, the exchange force between the memory layer M₁ and recording layer W₂ can be cut off, so that a change in the state of magnetization of the memory layer M₁ upon application of the initializing magnetic field Hi can be effectively prevented.

The exchange force control layer C₁₂, if it is made up of layer (2), is basically effective for decreasing the exchange force between the recording layer W₂ and the memory layer M₁. By the provision of such an exchange force control layer C₁₂, therefore, it is possible to prevent the reversal (initialization) of magnetization of the recording layer W₂ from having any adverse influence on the memory layer M₁.

Preferably, the exchange force control layer C₁₂ contains Gd, Fe, and Co as main components. Preferably in this case, the Gd content is in the range of 23 to 32 at %, and especially 24 to 30 at %. This range of the Gd content ensures a high C/N even upon repeated overwriting. When the layer (1) having too low a Gd content is used as the exchange force control layer C₁₂, the exchange force between this layer and other magnetic layer becomes too strong upon the easy axis of magnetization taking a vertical direction, rendering overwriting difficult. On the other hand, it is not preferable to construct the exchange force control layer C₁₂ of the layer (1) having too high a Gd content, because the exchange force between this layer and other magnetic layer becomes too weak when the easy axis of magnetization takes a vertical direction. The layer (1), when used as the exchange force control layer C₁₂, has an atomic ratio, Fe/(Fe+Co), of preferably 0.40 to 0.80, and more preferably 0.50 to 0.60. When the atomic ratio is too low, the exchange force becomes too weak when the easy axis of magnetization takes a vertical direction, and when the atomic ratio is too high, the Curie temperature becomes too low.

The thickness of the exchange force control layer C₁₂ is 30 nm or below, preferably 5 to 30 nm, and more preferably 8 to 20 nm. When the exchange force control layer C₁₂ is too thin, it is difficult to make use of the aforesaid action and thereby gain control of exchange force, and when the exchange force control layer C₁₂ is too thick, the exchange force between this layer and other magnetic layer becomes too weak.

Recording Layer W₂

The recording layer W₂ contains dysprosium or Dy, Fe, and Co as main components, and the rare earth element content of the recording layer W₂ is preferably 29 to 35 at %, and more preferably 30 to 33 at %. Too low a rare earth element content renders the initialization of the recording layer W₂ difficult, resulting in some considerable C/N drop upon overwriting. Too high a rare earth element content, on the other hand, gives rise to some considerable C/N drop, resulting in a much more reduced C/N upon overwriting.

The recording layer W₂ has an atomic ratio, Fe/(Fe+Co), of 0.40 to 0.58, and preferably 0.45 to 0.55. Too low an atomic ratio gives rise to a C/N drop whereas too high an atomic ratio results in some considerable C/N drop upon over-writing.

For the recording layer W₂, it is preferable to have a compensation temperature in a range of temperature that is higher than room temperature and lower than the temperature at which magnetization is transferred to the memory layer M₁. That is, the compensation temperature of the recording layer W₂ is preferably 100 to 160° C.

The thickness of the recording layer W₂ is preferably 15 nm or more, and more preferably 20 nm or more. When the recording layer W₂ is too thin, the exchange force between this layer and the memory layer M₁ becomes too strong for overwriting. On the other hand, no problem arises when the recording layer W₂ is thick. In view of production cost reductions, however, the upper limit on the thickness of the recording layer W₂ is preferably 60 nm, and especially 50 nm.

In embodiment I, it is preferable that the overall thickness of the magnetic multilayer structure comprising four magnetic layers should not exceed 80 nm. At a thickness exceeding 80 nm any high recording sensitivity is hardly available.

Embodiment II (FIGS. 2, 5, and 6)

A magneto-optical recording medium of FIG. 2 is similar to the magneto-optical recording medium of FIG. 1 with the exception that a recording layer W₂ is provided on its surface side with a switching layer S₃ and an initializing layer I₄ in the described order. The switching layer S₃ and initializing layer I₄ are each composed of an amorphous alloy containing a rare earth element and a transition element. Each layer is a magnetic layer having perpendicular magnetic anisotropy at room temperature. In FIG. 2, too, adjoining magnetic layers are mutually coupled by exchange force.

To perform the light intensity modulation overwriting to be explained just below, the following two conditions should be satisfied at the same time:

    Tc.sub.I4 >Tc.sub.W2 >Tc.sub.M1

    Tc.sub.I4 >Tc.sub.W2 >Tc.sub.S3

Here Tc_(M1) is the Curie temperature of a memory layer M₁, Tc_(W2) is the Curie temperature of the recording layer W₂, Tc_(S3) is the Curie temperature of the switching layer S₃, and Tc_(I4) is a Curie temperature of the initialization layer I₄. More preferably,

    Tc.sub.I4 >Tc.sub.W2 >Tc.sub.M1 >Tc.sub.S3

The light intensity modulation overwriting on the magneto-optical recording medium having such a multilayer construction as shown in FIG. 2 is explained with reference to FIGS. 5 and 6, where Tc_(I4) >Tc_(W2) >Tc_(M1) >Tc_(S3).

In FIGS. 5, and 6, [0] indicates a recording state where magnetic domains having upward magnetization are recorded in the recording layer M₁, and [1] indicates a recording state where magnetic domains having downward magnetization are recorded therein.

A medium capable of overwriting is placed in the recording state [0] or [1]. FIG. 5 is illustrative of the case where, on overwriting, the medium is put in the recording state [1] irrespective of the initial recording state, and FIG. 6 is illustrative of the case where, on overwriting, the medium is put in the recording state [0] irrespective of the initial recording state.

In the illustrated embodiment where the recording layer W₂ has a compensation temperature higher than room temperature, the direction of the overall magnetization is opposite to the direction of magnetization of the transition element subordinate lattice in the recording states [0] and [1]. In other magnetic layer, both arrows are in alignment with each other in the illustrated embodiment wherein it has no compensation temperature or a compensation temperature lower than room temperature.

Referring first to FIG. 5, the magnetic multilayer structure is irradiated with a laser beam to heat the area of the magnetic multilayer structure irradiated to a temperature that is higher than Tc_(W2) and below Tc_(I4), thereby allowing the magnetizations of the magnetic layers other than the initializing layer I₄ to vanish. As the laser beam moves, the temperature of the magnetic multilayer structure decreases. Upon the temperature falling below Tc_(W2) and being higher than the compensation temperature of the recording layer W₂, the recording layer W₂ is magnetized downward by a downwardly applied bias magnetic field Hb, and so the magnetization of the transition element subordinate lattice in the recording layer W₂ takes a downward direction. It is here to be noted that the bias magnetic field Hb is always applied for overwriting. Upon the temperature decreasing further and falling below Tc_(M1) and being higher than the compensation temperature of the recording layer W₂, the magnetization of the transition element subordinate lattice in the memory layer M₁ takes a downward direction by the exchange coupling force between the memory layer M₁ and the recording layer W₂, and so the magnetic multilayer structure is placed in the recording state [1]. As the temperature decreases further and reaches a temperature that is below Tc_(S3) and the compensation temperature of the recording layer W₂, magnetization occurs at the switching layer S₃, so that the direction of magnetization of the transition element subordinate lattice in the switching layer S₃ takes an upward direction by the exchange force between the switching layer S₃ and the memory layer M₁ and the magnetization of the transition element subordinate lattice in the recording layer W₂ is reversed in an upward direction by the exchange force between the recording layer W₂ and the switching layer S₃. To prevent a reversal of the magnetization of the memory layer M₁ at this time, the properties of each magnetic layer are predetermined in such a manner that, in this temperature range, the coercive force of the memory layer M₁ prevails over the exchange coupling force between the memory layer M₁ and the recording layer W₂. Furthermore, the properties of each magnetic layer are predetermined in such a manner that the exchange coupling force at the switching layer S₃ prevails over the influence of the bias magnetic field.

Referring to FIG. 6 the magnetic multilayer structure is irradiated with a laser beam whose power is lower than is the case with FIG. 5 to heat the magnetic multilayer structure to a temperature that is higher than Tc_(M1) and below Tc_(W2), thereby allowing the magnetizations of the memory layer M₁ and the switching layer S₃ to vanish. Then, as the temperature of the magnetic multilayer structure is below Tc_(M1), the magnetization of the transition element subordinate lattice in the memory layer M₁ takes an upward direction by the exchange force between the memory layer M₁ and the recording layer W₂, and so the magnetic multilayer structure is placed in the recording state [0]. As the temperature decreases further and falls below Tc_(S3), magnetization occurs at the switching layer S₃ ; upward magnetization of the transition element subordinate lattice occurs at the switching layer S₃ by the exchange coupling force between the switching layer S₃ and the initializing layer I₄ and the exchange coupling force between the switching layer S₃ and the recording layer W₂. In FIG. 6, too, the bias magnetic field Hb is always applied, as in the case of FIG. 5. However, this bias magnetic field Hb has no influence on the overwriting process of FIG. 6.

Whether in the case of high power recording of FIG. 5 or in the case of low power recording of FIG. 6, the magnetizations of the transition element subordinate lattices in the recording layer W₂, switching layer S₃ and initializing layer I₄ take an upward direction, and go back to their state before overwriting. In other words, the directions of the magnetizations of the recording layer W₂, switching layer S₃ and initializing layer I₄ other than the memory layer M₁ that determines the recording state are independent of hysteresis (overwriting). By irradiating the magnetic multilayer structure with a laser beam of high power or lower, therefore, it is possible to perform overwriting repeatedly. In other words, the light intensity modulation overwriting becomes feasible.

As can be see from FIGS. 5 and 6, each magnetic layer has the following role. The memory layer M₁ is a magnetic layer for storing the information to be read using the Kerr effect. The recording layer W₂ is a magnetic layer that plays a role in the magnetization of the memory layer M₁ by exchange coupling force, and determines the direction of magnetization of the memory layer M₁. The switching layer S₃ is a magnetic layer provided for cutting off a magnetic coupling between the recording layer W₂ and the initializing layer I₄ upon high power recording. As the switching layer S₃ is non-magnetized upon high power recording, the recording layer W₂ is magnetized in the direction of the bias magnetic field while it is not affected by the initializing layer I₄. The initializing layer I₄ is a magnetic layer that has always magnetization in one direction for the initialization of the recording layer W₂.

For simplicity of illustration, the magnetic multilayer structure is assumed to be heated to a temperature higher than Tc_(W2) and below Tc_(I4) in the case of FIG. 5. However, if the recording layer W₂ can practically be in alignment with the direction of the bias magnetic field, the target temperature can then be below Tc_(W2). Likewise, the magnetic multilayer structure is assumed to be heated to a temperature higher than Tc_(M1) and below Tc_(W2) in the case of FIG. 6. However, if the magnetization of the recording layer W₂ can be transferred to the memory layer M₁, the target temperature can then be below Tc_(M1).

In FIGS. 5 and 6, the condition Tc_(I4) >Tc_(W2) >Tc_(M1) >Tc_(S3) is assumed to hold. However, there is neither upper nor lower limit on Tc_(M1) and Tc_(S3), as already noted. In other words, the condition Tc_(S3) >Tc_(M1) may also hold unlike FIGS. 5 and 6. If, in this case, the exchange force between the switching layer S₃ and the recording layer W₂ remains small by the time the magnetization of the recording layer W₂ is transferred to the memory layer M₁, no problem arises because no reversal of the magnetization of the recording layer W₂ occurs even when magnetization occurs at the switching layer S₃. If the switching layer S₃ has a composition (compensation temperature composition) whose compensation temperature lies in the vicinity of room temperature, the exchange energy of the switching layer S₃ then increases at a further decreased temperature. Thus, the magnetization of the recording layer W₂ is reversed (initialized) by the exchange force between the this layer and the switching layer S₃ ; overwriting independent of hysteresis becomes feasible as in FIGS. 5 and 6.

Magnetic Multilayer Structure

Readout Layer R₀₁ Memory Layer M₁, and Exchange Force Control Layer C₁₂

These magnetic layers have the same constructions as in embodiment I.

When the exchange force control layer C₁₂ is made up of the layer (1) as mentioned above, the easy axis of magnetization thereof takes a vertical direction. In FIGS. 5 and 6, therefore, transfer of magnetization from the recording layer W₂ to the memory layer M₁ by exchange force takes place easily. Then, the magnetization of the recording layer W₂ is reversed (initialized) by the initializing layer I₄ via the switching layer S₃. Since, at this time, the easy axis of magnetization of the exchange force control layer C₁₂ is in its own plane direction, the exchange force between the memory layer M₁ and the recording layer W₂ can be cut off, so that a change in the state of magnetization of the memory layer M₁ can be effectively prevented.

Recording Layer W₂

The recording layer W₂ in embodiment II contains Dy, Fe, and Co as main components, and the rare earth element content of the recording layer W₂ is preferably 29 to 35 at %, and more preferably 30 to 33 at %. Too low a rare earth element content renders the initialization of the recording layer W₂ for high power recording difficult, resulting in some considerable C/N drop upon overwriting. Too high a rare earth element content, on the other hand, causes the compensation temperature of the recording layer W₂ to disappear, giving rise to some considerable C/N drop, and resulting in a much more reduced C/N upon overwriting.

The recording layer W₂ has an atomic ratio, Fe/(Fe+Co), of 0.40 to 0.58, and preferably 0.45 to 0.55. Too low an atomic ratio gives rise to a C/N drop whereas too high an atomic ratio results in some considerable C/N drop upon over-writing.

For the recording layer W₂, it is preferable to have a compensation temperature in a range of temperature that is higher than room temperature and lower than the temperature at which magnetization is transferred to the memory layer M₁. That is, the compensation temperature of the recording layer W₂ is preferably 100 to 160° C.

The recording layer W₂ is otherwise the same as that in embodiment I.

Switching Layer S₃

The switching layer S₃ contains Tb, and Fe as main components. The Tb content of the switching layer S₃ is in the range of preferably 23 to 29 at %, and more preferably 24 to 27 at %. Too little or much Tb causes exchange coupling to become weak due to a Curie temperature drop and a saturation magnetization increase.

The switching layer S₃ has a thickness of preferably 5 to 15 nm, and more preferably 8 to 12 nm. When the switching layer S₃ is too thin, the cutoff of exchange force between the recording layer W₂ and the initializing layer I₄ becomes insufficient. When the switching layer S₃ is too thick, the exchange forces between this layer and the recording layer W₂ and this layer and the initializing layer I₄ become too small for the initialization of the recording layer W₂.

Initializing Layer I₄

The initializing layer I₄ contains Tb, and Co as main components. The Tb content of the initializing layer I₄ is in the range of preferably 21 to 28 at %, and more preferably 23 to 27 at %.

The initializing layer I₄ has a thickness of preferably 15 to 50 nm, and more preferably 18 to 45 nm. When the initializing layer I₄ is too thin, the exchange force between this layer and the recording layer W₂ becomes too large for keeping the magnetization of the initializing layer I₄ in one direction, because spin reversal is likely to occur. No particular problem arises when the initializing layer I₄ is thick. However, a thickness exceeding 50 nm is not necessary because of a film formation cost increase.

In both embodiments I and II, each magnetic layer may additionally contain a rare earth element other than that referred to as the main component. By the rare earth element used herein are intended yttrium or Y, scandium or Sc, and lanthanide elements. Each magnetic layer may also contain a transition element other than that referred to as the main component.

Substrate

To write or read information on or out of a magneto-optical recording medium, the back side (the side of the memory layer M₁) of the substrate is irradiated with laser light. To this end, it is preferable that the substrate is transparent to laser light (of about 400 to 900 nm wavelength). For instance, transparent resins such as polycarbonate, acrylic resin, amorphous polyolefin, and styrene resin or glasses may be used for the substrate.

First, and Second Dielectric Layers

The first, and second dielectric layers are provided for the purposes of improving C/N, and preventing corrosion of the magnetic layers. When the heat radiating layer is provided, the second dielectric layer behaves not only as an accumulator for heat generated from the recording layer during recording but also as a transmitter for transmitting the heat to the heat radiating layer.

Preferably, the first dielectric layer has usually a thickness of 30 to 100 nm, and the second dielectric layer has usually a thickness of 5 to 100 nm.

Each dielectric layer may be constructed of an oxide or nitride or their mixture, for instance, silicon oxide, silicon nitride, aluminum nitride, and SiAlON.

Heat Radiating Layer

The heat radiating layer, provided as occasion arises, acts to radiate heat from the magnetic multilayer structure during recording. The heat radiating layer has usually a thickness of 20 to 80 nm although the invention is not limited thereto.

The heat radiating layer is constructed of a metal (inclusive of an alloy). Preferably, the heat radiating layer is composed of Al, Au, Ag, and Cu or an alloy containing at least one thereof, to which suitable amounts of additive elements such as Ni, Ti, Cr, Zn, and Co may be added.

Protective Layer

Preferably, the heat radiating layer is provided on its surface with a protective layer formed of a resin such as ultraviolet radiation curing resin. The protective layer has preferably a thickness of 1 to 30 μm. Such a protective layer may also be provided on the back side of the substrate.

EXAMPLE Example 1 (Embodiment II)

For a substrate, a disk form of polycarbonate of 120 mm in outer diameter and 1.2 mm in thickness was used (with a track pitch of 1.1 μm). Using a second dielectric layer and a heat radiating layer, each with a thickness shown in Table 2, a magneto-optical recording disk sample No. 1 having such a multilayer construction as shown in FIG. 2 was prepared in the following procedure.

First Dielectric Layer

Using an Si target, a silicon nitride film was formed by a sputtering process in an Ar plus N₂ atmosphere to obtain a first dielectric layer of 60 nm in thickness.

Each Magnetic Layer in Magnetic Multilayer Structure

Each magnetic layer was prepared by a sputtering process in an Ar atmosphere.

Second Dielectric Layer

A second dielectric layer of 10 nm in thickness was prepared as in the first dielectric layer.

Heat Radiating Layer

Using an Al-Ni alloy target, a heat radiating layer was prepared by a sputtering process in an Ar atmosphere.

Protective Layer

An ultraviolet radiation curing resin was applied by a spin coating process on the heat radiating layer, and then irradiated with ultraviolet radiation for curing, thereby obtaining a protective layer of about 5 μm in thickness.

The composition, thickness, and Curie temperature (Tc) of each magnetic layer are indicated in Table 1. Compensation temperatures (Tcomp), which some magnetic layers have, are also indicated. It is here to be noted that the compositions of the magnetic layers were determined with an Auger analyzer after their properties had been estimated as will be described layer. The thickness of each magnetic layer was found from the sputtering rate and time. The sputtering rate was determined from the actually found thickness of a thick film formed by long-term sputtering under the same conditions as actually applied, and the sputtering time.

                  TABLE 1                                                          ______________________________________                                                    Composition                                                                               Thickness Tc    Tcomp                                      Magnetic Layer at % nm ° C. ° C.                               ______________________________________                                         Readout Layer R.sub.01                                                                    Gd.sub.25 Fe.sub.53 Co.sub.22                                                             10        >300  --                                         Memory Layer M.sub.1 Tb.sub.23 Fe.sub.69 Co.sub.8 15 180 --                    Exchange Force Gd.sub.27 Fe.sub.52 Co.sub.21 10 >300 150                       Control Layer C.sub.12                                                         Recording Layer W.sub.2 Dy.sub.31 Fe.sub.35 Co.sub.34 30 280 120                                                    Switching Layer S.sub.3 Tb.sub.25                                             Fe.sub.75 10 150  20                       Initializing Tb.sub.26 Co.sub.74 40 >300 180                                   Layer I.sub.4                                                                ______________________________________                                    

Then, a number of samples, shown in Table 2, were prepared as in sample No. 1 with the exception that the rare earth element contents, and thicknesses of the readout layer R₀₁, memory layer M₁, and exchange force control layer C₁₂ were changed as shown in Table 2. In this regard, it is to be noted that the Fe to Co ratio was held at the same value as in sample No. 1 irrespective of the rare earth element content of each magnetic layer. The rare earth element contents shown in Table 2 are the Gd content for the readout layer R₀₁, the Tb content for the memory layer M₁, and the Gd content for the exchange force control layer C₁₂.

Estimation of Properties

The properties of each sample were estimated using an optical disk estimating device under the following conditions.

Laser Wavelength: 680 nm

Numerical Aperture NA: 0.55

Recording Power: 13 mW for high power recording 4 mW for low power recording

Read Power: 1.5 mW

Bias Magnetic Field: 300 Oe

Relative Linear Velocity: 7.4 m/s

Recording Pattern: pulse division mode 20 ns (on)-152 ns (off)

C/N values determined after 10,000 overwriting cycles are shown in Table 2.

                                      TABLE 2                                      __________________________________________________________________________           Rare Earth              C/N (dB)                                           Sample Element (at %) Thickness (nm) t.sub.R01 / after over-                 No.   R.sub.01                                                                          M.sub.1                                                                           C.sub.12                                                                          R.sub.01                                                                          M.sub.1                                                                           C.sub.12                                                                          (t.sub.R01 + t.sub.M1)                                                               writing cycles                                   __________________________________________________________________________      1    25 23 27 10 15 10 0.4   49                                                  2 23 23 27 10 15 10 0.4 48                                                     3 27 23 27 10 15 10 0.4 47                                                     4 25 20 27 10 15 10 0.4 47                                                     5 25 24 27 10 15 10 0.4 48                                                     6 25 23 24 10 15 10 0.4 47                                                     7 25 23 30 10 15 10 0.4 47                                                     8 25 23 27  8 17 10 0.32 46                                                    9 25 23 27 13 12 10 0.52 46                                                   10 25 23 27 16 20 10 0.44 46                                                   11 25 23 28 10 20 25 0.4 47                                                    12 (comp.)  22* 23 27 10 15 10 0.4 43                                          13 (comp.)  28* 23 27 10 15 10 0.4 42                                          14 (comp.) 25  19* 27 10 15 10 0.4 40                                          15 (comp.) 25  27* 27 10 15 10 0.4 42                                          16 (comp.) 25 23  22* 10 15 10 0.4 41                                          17 (comp.) 25 23  33* 10 15 10 0.4 41                                          18 (comp.) 25 23 27  6 19 10 0.24* 44                                          19 (comp.) 25 23 27 17  8 10 0.68* 41                                          20 (comp.) 25 23 27  20*  25* 10 0.44 42                                       21 (comp.) 25 23 27 10 15  40* 0.4 40                                        __________________________________________________________________________      *deviations from the scope of the invention                              

Table 2 clarifies the advantages of the invention. In the inventive samples, high C/N values are obtained even after 10,000 overwriting cycles due to high recorded information storage stability. This is in sharp contrast to the comparative samples wherein much lower C/N values are obtained after 10,000 overwriting cycles because the rare earth element content, and thickness of the readout layer R₀₁, memory layer M₁, and exchange force control layer C₁₂ deviate from the scope of the invention.

Example 2 (Embodiment I)

A magneto-optical recording disk sample having such a multilayer construction as shown in FIG. 1 was prepared as in sample No. 1 of example 1 with the exception that the composition (in atomic ratio) of the recording layer W₂ was changed to Dy₃₂ Fe₃₄ Co₃₄, with no provision of the switching layer S₃ and initializing layer I4. The C/N of this sample was measured as in example 1. For this measurement, however, a magnetic field of 3 kOe was applied as the initializing magnetic field. The initial C/N value was 48 dB, and the C/N value after 10,000 overwriting cycles was again 48 dB. Thus, there was no degradation.

The results of each example clarify the advantages of the invention.

Japanese Patent Application 137859/1997 is incorporated herein by reference.

Although some preferred embodiments have been described, many modifications and variations may be made thereto in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. 

What we claim is:
 1. A magneto-optical recording medium capable of direct overwriting in a light intensity modulation mode, which comprises a substrate and a magnetic multilayer structure located on a surface side thereof, wherein:said magnetic multilayer structure comprises, in order from a substrate side thereof, a readout layer R₀₁, a memory layer M₁, an exchange force control layer C₁₂ and a recording layer W₂, each of said magnetic layers containing a rare earth element and a transition element, said readout layer R₀₁, said memory layer M₁, and said recording layer W₂ having perpendicular magnetic anisotropy at room temperature, and adjacent magnetic layers being mutually coupled with exchange force, said magnetic multilayer structure satisfying the following conditions:

    Tc.sub.W2 >Tc.sub.M1

    t.sub.R01 /(t.sub.R01 +t.sub.M1)=0.3 to 0.6

    t.sub.R01 +t.sub.M1 =20 to 40 nm

where Tc_(M1) is a curie temperature of said memory layer M₁, Tc_(W2) is a Curie temperature of said recording layer W₂, t_(R01) is a thickness of said readout layer R₀₁, and t_(M1) is a thickness of said memory layer M₁, said readout layer R₀₁ containing gadolinium, iron, and cobalt as main components with the proviso that gadolinium is contained in an amount of 23 to 27 at %, said memory layer M₁ containing terbium, iron, and cobalt as main components with the proviso that terbium is contained in an amount of 21 to 25 at %, and said exchange force control layer C₁₂ containing gadolinium, iron, and cobalt as main components with the proviso that gadolinium is contained in an amount of 23 to 32 at %, and having a thickness of 30 nm or below.
 2. The magneto-optical recording medium of claim 1, wherein said recording layer W₂ contains dysprosium, iron, and cobalt as main components.
 3. The magneto-optical recording medium of claim 1, wherein said magnetic multilayer structure has a thickness of up to 80 nm.
 4. The magneto-optical recording medium of claim 1, wherein:said magnetic multilayer structure further comprises, in order from a surface side of said recording layer W₂, a switching layer S₃ and an initializing layer I₄, each being a magnetic layer containing a rare earth element and a transition element and having perpendicular magnetic anisotropy at room temperature, adjacent layers in said magnetic multilayer structure are mutually coupled with exchange force, and said magnetic multilayer structure satisfy simultaneously the following conditions:

    Tc.sub.I4 >Tc.sub.W2 >Tc.sub.M1

    Tc.sub.I4 >Tc.sub.W2 >Tc.sub.S3

where Tc_(M1) is the Curie temperature of said memory layer M₁, Tc_(W2) is the Curie temperature of said recording layer W₂, Tc_(S3) is a Curie temperature of said switching layer S₃, and Tc_(I4) is a Curie temperature of said initializing layer I₄.
 5. The magneto-optical recording medium of claim 4, wherein said switching layer S₃ contains terbium, and iron as main components, and said initializing layer I₄ contains terbium, and cobalt as main components. 